Piezoelectric accelerometer

ABSTRACT

A piezoelectric accelerometer in which a measuring element is mounted in a housing of ceramic material with a part adapted to rest on the object to be measured to achieve a high degree of heat resistance together with heat insulation and good transfer of mechanical oscillations.

O United States Patent 1 51 3,636,387 Hatschek 1451 Jan. 18, 1972 4]PIEZOELECTRIC ACCELEROME'IER 34mm CM [72] Inventor: Rudoll A. lhtachek,Fribourg, Switzerland UNITED STATES PATENTS [73] Assignee: Vlbro-MeterAG, Fribourg, Moncor, Swit- 3,378,705 4/1968 Bacon ..310/9.5 zerland 3390,287 6/ l 968 Sondereggew ....3 l0/9.6

3,46l,327 8/1969 Zeiringer ....3l0/8.9 [221 3,042,144 7/1962- Shoot....310/s.4 211 Appl.No.: 836,098 3,229,153 1/1966 Freeman "315/55 1Primary Examiner--D. F. Duggan [30] Appumuo Priority D." AssistantExaminer-B. A. Reynolds July 4, 1968 Austria ..A 6402/68 Manley-lawn,Cole, Grindle & Watson 521 u.s.1*1 11o/a.4,310/9 1 ABSTRACT {25}iiifilisa m 1.1:1:1:111111111111111111115'15751;323:2332}?! A which ismounted in a housing of ceramic material with a part adapted to rest onthe object to be measured to achieve a high degree of heat resistancetogether with heat insulation and good transfer of mechanicaloscillations.

5 Claims, 3 Drawing Figures PIEZOELECTRIC ACCELEROMETER This inventionrelates to a piezoelectric accelerometer whose measuring element isdisposed in a housing which is mounted on the object which is to bemeasured.

Piezoelectric accelerometers are used as it is well known, for themeasuring of accelerating forces and especially for the surveillance ofvibrations of running machines and driving systems, for example, for theoperational surveillance of airplane power units. ln order to makepossible a good transfer of the vibrations of the supervised engines tothe sensitive measuringelementof the accelerometer, the housing of theaccelerometer must be in a rigid mechanical connection with thesupervised engine and therefore it must be mounted at suitable positionsdirectly on the engine. The accelerometers in that case are exposedoften to relatively high temperatures, as for example, in the case ofthe surveillance of jet engines, in which case they are seated directlyon the casing of the propulsion system. The piezoelectric quartz bodiesused most frequently for practical purposes can be used only up to atemperature of 400-500 C., because at a greater heating up of thequartz, thesignals become too weak. Further difficulties result in thecase of higher temperatures by the heat expansions of the housing whichoccur and of the sensitive measuring element which because of thevarying heating up, often differ from one another and can lead to adisadvantageous change in the bias power of the piezoelement.

Thus two essential requirements are made above all, in regard topiezoelectric accelerometers, that is the mechanical connection betweenthe measured object, and the sensitive measuring element arranged in thehousing of the accelerometer, must be as rigid as possible in order thata good transference of the oscillations and vibrations can beaccomplished, and the component parts of the accelerometer must have asufficient heat resistance without as a result thereof, impairing themeasurement technical characteristics of the accelerometer especiallyits sensitiveness. The piezoelectric accelerometers known hitherto donot always meet these requirements.

This invention has for its object and improvement of the piezoelectricaccelerometers both in regard to heat resistance and in the transfer ofoscillations, and it consists in the fact that the housing is made of aceramic material preferably of aluminum oxide ceramic material, at leastin part and at the very least, at that end which rests on the object tobe measured. It has turned out that ceramic materials do not merely haveadvantageous mechanical and thermal characteristics but that they alsomake possible a suitable shaping without difficulty and that they can beassembled with the remaining components of the accelerometer. Forexample ceramic material made of aluminum oxide has a considerablyhigher elasticity modulus than steel, as a result of which, a goodtransference of mechanical signals will be assured and besides, a highdegree of heat resistance so that an accelerometer made out of it canwithstand extreme temperatures. Furthermore, as is well known, ceramicmaterials are poor heat conductors, so'that a further advantage of theinvention consists in a decrease in the heat conduction between theobject to be measured and the sensitive measuring element.

Ceramic materials hitherto have been used in connection withpiezoelectric accelerometers only for purposes of electric insulationand during calibration of the accelerometer as a support for the jointreception of the calibrating device and of the accelerometers that areto be calibrated. in the case of a known design, the bottom surface ofthe metallic housing of the accelerometer, with which the latter ismounted on the ob ject to be measured, is provided with a thin layermade of ceramic material in order to achieve an electric insulationbetween the accelerometer and the object to be measured. However, as aresult thereof, neither the transference of mechanical signals will beimproved essentially nor will the heating up of the accelerometer belowered or its heat resistance improved. Also in the case of calibrationof accelerometers, the use of ceramic materials has for its purpose inthe first place to establish an electric insulation between the housingsof the calibration device consisting in a customary manner of metal, andof the accelerometers that are to be calibrated on the one hand, andbetween said accelerometers and the vibrating table supplying 'thecalibration signal on the other hand. Furthermore, inaccuracies duringcalibration based on varying oscillations between the transmitter thatis to be tested and the calibration device should be avoided by theconsiderable rigidity of the ceramic materials. Difiering from theseknown uses, thehousing of the accelerometer itself accordingto theinvention is made wholly or partially of ceramic material and besides arigid mechanical connection between the accelerometer and the object tobe measured, a high degree of heat resistance of the accelerometer willbe achieved so that the latter can also be used at measuring placeswhere high temperaturesoccur.

Within the framework of the invention various designs of theaccelerometer have proven themselves to be advantageous. For example,the housing may have a lower part made of ceramic material with a hollowspace which is open on top and which is closed by an upper partconsisting of metal and placed onto thelower part whereby the lower partand the upper part together encompass the measuring element. The housinghowever can consist of ceramic material. It can possibly have an annularbase on which an intermediate piece made of metal is attached preferablyby hard soldering to which the metallic upper part of the housingencompassing the measuring element has been welded on. In both casesthat part of the housing which is thermally under the highestload'across which the transference of the mechanical signals also takesplace, consists of ceramic material whereas the part of the housingwhich is under a lesser load is produced in the customary manner ofmetal. Thus, one is dealing with simple fonns of design of theaccelerometer according to the invention.

In order to shield the sensitive measuring element electrically andmagnetically, it is possible according to the invention to provide theparts of the housing consisting. of ceramic material, which encompassesthe measuring element, with a metallic coating provided preferably onthe inside wall of the housing By this practical measure, above all inthe case of the housing produced by ceramic material in their entirety,electric and magnetic interspersions are avoided by which themeasurement could be invalidated.

Further objects of the invention will be apparent from the followingdescription when considered in connection with the accompanying drawingin which,

FIG. 1 is an axial section of the accelerometer according to theinvention,

FIG. 2 is an axial section of a modified embodiment, and

FIG. 3 is an axial section of a further modified embodiment.

in all embodiments given by way of example, the sensitive measuringelement 1 of the accelerometer has been disposed in a housing 2, whichis mounted with its bottom surface 3 directly on the object that is tobe measured. As becomes clear from FIG. 3, the measuring element 1consists of disks 4 made of piezoelectric material which has been placedin layers above one another with interpositionof electrodes and of aseismic mass 5. The bias of the measuring element is accomplished by abiasing arrangement which, according to FIG. 1, consists of a bias coil6 and in the embodiments according to FIGS. 2 and 3, shown by way ofexample, of a bias plate spring 7. The electric connecting lines 8"areconducted to the outside in an airtight manner through a connecting plug9 attached to the housing 2. The seismic mass 5 shown in FIG. 3 issupported rigidly on the bias plate spring 7 by means of a ball 10. Theattachment of the accelerometer to the place of measurement isaccomplished by means of screws which penetrate bores 11 in the floorpart of the housing 2 which can be developed, for example, in the mannerof a flange with a triangular base surface.

In the case of the embodiment shown in F IG. 1 by way of example, thehousing 2 has a lower part 12 made from ceramic material preferably fromaluminum oxide ceramic material which lower part has been provided witha hollow space 13 open on top into which the measuring element 1 isinserted from above. The upper part of the housing 2 consists of metaland is composed of a bias casing 6 and a cover hood [4. According toFIG. 2 on the other hand, the housing 2 is made in its entirety ofceramic material and has been equipped with a cylindrical hollow spacewhich houses the measuring element 1. The bias plate spring 7 has beeninserted into the hollow space 15 and has been attached with therequired pretension. In the parts of the housing 2 consisting of ceramicmaterial, the hollow spaces 13 and 15 are provided with a metalliccoating 16 which has been applied to the inside wall of the housing, forexample, it has been sprayed on or steamed on and it has the object ofshielding the sensitive measuring element 1 electrically andmagnetically.

In the embodiment according to FIG. 3, given by way of example, thehousing 2 has been provided with an annular base 17 consisting ofceramic material on which an intermediate piece 19 made of metal hasbeen attached by means of a layer of solder 18, on which has been placedan upper part 20 of the housing consisting of metal in the customarymanner. A firm connection between the intermediate piece 19 and theupper part 20 is accomplished by means of a welding seam 21. Themetallic upper part 20 will receive the sensitive measuring element 1 ina cylindrical hollow space 22, and it forms the shield at the same timeso that a metallic coating is superfluous. The

prestressed plate spring 7 has been inserted into the cylindrical hollowspace 22 and has been attached therein in the prestressed position, by awelding seam 23. 7 1

In all embodiments given by way of example, the housing 2 of theaccelerometer has thus been produced from ceramic material, at leastpartially and to be sure at the very least, that part of the housingwhich rests directly on the object to be measured. In consequence of thehigh modulus of elasticity of the ceramic material, a rigid mechanicalconnection between the object to be measured and the sensitive measuringelement of the accelerometer will be assured as a result thereof.Besides, the accelerometer excells under great heat resistance since thehousing consists of ceramic material precisely in those parts which areexposed to the highest temperature, which ceramic material has a greatheat resistance and a low heat conductivity. Finally, the shapes showncan be easily produced.

lclaim:

l. A piezoelectric accelerometer comprising a housing adapted to bemounted on the object that is to be measured, a measuring element in thehousing, the housing being composed of a ceramic material at leastpartially and at least to that part of the housing which is adapted torest directly on the object to be measured, to avoid metallic heattransmission between the said object and the measuring element and toachieve a higher degree of heat resistance, heat insulation and goodtransfer of mechanical oscillations.

2. Piezoelectric accelerometer according to claim 1, in which thehousing has a lower part with a hollow space open on top and which isclosed at an upper part consisting of metal and placed on the lower partwhereby the lower part and the upper part together enclose the measuringelement.

3. Piezoelectric accelerometer according to claim 1, in which thehousing has an annular base consisting of ceramic material on which byintermediate element made of metal is attached to which themetallicupper part of the housing encompassing the measuring element hasbeenwelded.

4. Piezoelectric accelerometer according to claim 1, in which theceramic material is produced from an aluminum oxide material.

5. Piezoelectric accelerometer according to claim 3, in which theintermediate element is attached by soldering.

1. A piezoelectric accelerometer comprising a housing adapted to bemounted on the objecT that is to be measured, a measuring element in thehousing, the housing being composed of a ceramic material at leastpartially and at least to that part of the housing which is adapted torest directly on the object to be measured, to avoid metallic heattransmission between the said object and the measuring element and toachieve a higher degree of heat resistance, heat insulation and goodtransfer of mechanical oscillations.
 2. Piezoelectric accelerometeraccording to claim 1, in which the housing has a lower part with ahollow space open on top and which is closed at an upper part consistingof metal and placed on the lower part whereby the lower part and theupper part together enclose the measuring element.
 3. Piezoelectricaccelerometer according to claim 1, in which the housing has an annularbase consisting of ceramic material on which by intermediate elementmade of metal is attached to which the metallic upper part of thehousing encompassing the measuring element has been welded. 4.Piezoelectric accelerometer according to claim 1, in which the ceramicmaterial is produced from an aluminum oxide material.
 5. Piezoelectricaccelerometer according to claim 3, in which the intermediate element isattached by soldering.